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The GDT is a table of 8-byte entries. Each entry may refer to a segment descriptor, Task State Segment (TSS), Local Descriptor Table (LDT), or call gate.Call gates were designed for transferring control between x86 privilege levels, although this mechanism is not used on most modern operating systems.
The descriptors may be either interrupt gates, trap gates or, for 32-bit protected mode only, task gates. Interrupt and trap gates point to a memory location containing code to execute by specifying both a segment (present in either the GDT or LDT) and an offset within that segment. The only difference between trap and interrupt gates is that ...
A segment register called the task register (TR) holds a segment selector that points to a valid TSS segment descriptor which resides in the GDT (a TSS descriptor may not reside in the LDT). Therefore, to use a TSS the following must be done by the operating system kernel: Create a TSS descriptor entry in the GDT
The original GDT_TS is calculated based on the superimpositions and GDT scores produced by the Local-Global Alignment (LGA) program. [1] A "high accuracy" version called GDT_HA is computed by selection of smaller cutoff distances (half the size of GDT_TS) and thus more heavily penalizes larger deviations from the reference structure.
The LDT is used for a particular task. But it can be shared between the tasks as well. GDT contains a descriptor pointing to the base of the LDT. The register LDTR points to the LDT descriptor within the GDT. A LDT descriptor copy is stored in 64-bit register in CPU. (fig)
There are two such tables, the Global Descriptor Table (GDT) and the Local Descriptor Table (LDT), each holding up to 8192 segment descriptors, each segment giving access to 64 KB of memory. In the 80286, a segment descriptor provides a 24-bit base address , and this base address is added to a 16-bit offset to create an absolute address.
The next bit (bit 2) specifies whether the operation is used with the GDT or the LDT. The lowest two bits (bit 1 and bit 0) of the selector are combined to define the privilege of the request, where the values of 0 and 3 represent the highest and the lowest privilege, respectively.
LCNLG – LDT CNL gamma ray log; LCR – local control room; LCV – level control valve; L/D – lay down (such as tubing or rods) LD – lay down (such as tubing or rods) LDAR – leak detection and repair; LDHI – low-dosage hydrate inhibitor; LDL – litho density log; LDS – leak detection system (pipeline monitoring) LDTEP – LDT EPT ...